Pseudomonas Capsulatum A12 Reduces Cadmium Accumulation In Hydroponic Rice (Oryza Sativa L.) Seedlings

The heavy metal cadmium（Cd）in paddy fields in China is seriously polluted,and it is easy to cause a series of health problems after entering the human body through the food chain.Therefore,it is particularly urgent to effectively restore the heavy metal pollution in the environment.Although many schemes have been proposed for Cd pollution remediation,there is no economical and effective treatment method.Microbial remediation technology provides a new possibility for efficient treatment of heavy metal pollution because of its advantages of low cost,simple operation and small impact on the environment.In the early stage,a photosynthetic bacterium named A12 was isolated from the soil polluted by Cd and identified as Rhodopseudomonas capsulatum,which was grown in the medium with different Cd²⁺concentration.The maximum tolerance concentration of A12 to Cd²⁺was 200.0 mg/L.The effect of four independent variables,initial Cd²⁺ions concentration（10.0 mg/L～200.0 mg/L）,p H（4～8）,biosorbent dosage（0.01 g/L～1.0 g/L）,and time（1 min～120 min）on Cd²⁺ions biosorption was examined.X-ray photoelectron spectroscopy（XPS）was applied to determine the surface functional groups that are responsible for Cd adsorption.The effects of A12 on the growth and Cd²⁺accumulation of Oryza sativa L.seedlings were observed by applying A12 to hydroponic Oryza sativa L.seedlings.The results showed that the adsorption capacity of A12 on Cd²⁺increased with the increase of initial concentration of Cd²⁺,and the adsorption capacity reached208.8 mg/g when the concentration was 200.0 mg/L.The effect of bacteria dosage on adsorption capacity increased firstly and then decreased,and the adsorption capacity was the highest when the bacteria dosage was 0.5 g/L.p H has a great influence on the adsorption of Cd²⁺,and the optimal adsorption p H condition is p H=7.Through kinetic simulation,it was found that a pseudo second order kinetic model is closer to describing the Cd²⁺biosorption process of A12,and it confiems that chemisorption was the rate-limiting step.Using XPS spectra,analysis showed that the hydrocarbon groups,amide groups and protonated amino groups on A12 surface participated in the adsorption process of Cd²⁺.At the same time,A12 could promote the growth of Oryza sativa L.seedlings and reduce the accumulation of Cd²⁺in Oryza sativa L.seedlings.New findings of this research suggest that P.capsulatum,as a biosorbent,could adsorb Cd²⁺and effectively reduce the accumulation of Cd²⁺in Oryza sativa L.seedlings.This study provides a new idea for the bioreremediation of heavy metal contaminated soil by P.capsulatum,and also provides theoretical support for the adsorption mechanism of heavy metals by microorganisms.